Conventionally, with respect to a developing system for an electrostatic latent image formed on an image supporting member in an image-forming apparatus using an electrophotographic system, a one-component developing system that uses only toner as a developer and a two-component developing system that uses a toner and a carrier have been known.
In the one-component developing system, in general, the toner is allowed to pass through a regulating section that is constituted by a toner-supporting member and a regulating plate pressed onto the toner-supporting member so that the toner is charged and a desired thin toner layer is obtained; therefore, this system is advantageous from the viewpoints of simplifying and miniaturizing the system and of achieving low costs. In contrast, due to a strong stress in the regulating section, the toner is easily deteriorated to cause degradation in a toner charge-receiving property. Moreover, the regulating plate and the surface of the toner-supporting member are contaminated by the toner and externally additive agents, with the result that a charge-applying property to the toner is lowered to cause problems such as fogging and the subsequent short service life of the developing system.
In comparison with the one-component developing system, the two-component developing system, which charges the toner through a friction-charging process upon mixing with the carrier, can reduce the stress, and is advantageous in preventing toner deterioration. Moreover, the carrier serving as a charge-applying material to the toner has a greater surface area so that it is relatively resistant to contamination due to the toner and externally additive agents, and is advantageous in prolonging the system service life. However, even in the case of the two-component developer, the contamination on the carrier surface due to the toner and externally additive agents also occurs to cause reduction in the quantity of charge in toner after a long-term use, resulting in problems such as fogging and toner scattering; therefore, the system service life is not sufficient, and there is a strong demand for a longer service life.
To meet the demand, a few technologies for prolonging the life of the two component developer by preventing the carrier from deteriorating have been proposed (for example, Japanese Patent Application Laid-Open Publication No. 59-100471 and Japanese Patent Application Laid-Open Publication No. 2003-215855)
With respect to a method for prolonging the life of the two component developer, Japanese Patent Application Laid-Open Publication No. 59-100471 has disclosed a development apparatus in which a carrier, alone or together with a toner, is supplied little by little, while a deteriorated developer having a reduced electrostatic charge property (simply referred to as “charge property”) is discharged in response to a supply so that the carrier is exchanged to prevent increase in the a ratio of a deteriorated carrier. In this device, since the carrier is exchanged, a reduction in the quantity of charge in toner due to the deteriorated carrier can be suppressed in a certain level, making it possible to provide a long service life. However, since a mechanism for collecting the discharged carrier is required, and since the carrier is used as a consumable supply, problems arise in costs, environmental preservation, and the like. Moreover, since a predetermined number of printing processes need to be repeated until a ratio of new and old carriers has been stabilized, there is a failure to maintain and effectively use the initial properties.
Japanese Patent Application Laid-Open Publication No. 2003-215855 has disclosed a two component developer composed of a carrier and a toner to which particles that exert a charge property with a reverse polarity to a toner charge polarity are externally added, and a developing method using such a developer. In the developing method of Japanese Patent Application Laid-Open No. 2003-215855, the reverse polarity-chargeable particles are added in an attempt to add functions as a polishing agent and spacer particles, and it describes that by the effect of removing spent matters on the carrier surface, the degradation preventive effect is obtained. Moreover, it also describes that in a cleaning unit in an image supporting member, a cleaning property is improved, and that a polishing effect of the image supporting member is obtained. However, in the disclosed developing method, the amounts of consumption in the toner and the reverse polarity-chargeable particles are different depending on an image area rate, and in particular, in a case of a small image area rate, the consumption of the reverse polarity-chargeable particles becomes excessive, causing degradation in a carrier deterioration preventive effect in a development apparatus.